The present invention relates to acoustic reproduction devices and more particularly to sound radiator systems comprising means for radiating acoustic energy to a given listening position or listening area within a listening room, such that undesired reflections experienced at the listening position and within the said listening area originating from for instance room boundaries or from specific surfaces of obstacles in the room can be either avoided altogether or at least attenuated in a controlled manner.
In designing loudspeakers and systems of loudspeakers both for domestic and professional use, one important acoustical characteristic of such systems is the directivity of radiation of acoustic energy to the surroundings. Generally sound is not only radiated directly towards the listening position in the listening room but also towards the various boundaries of the room and towards different objects present in the room. When sound impinges on such boundaries at least a part of the acoustic energy is reflected from the boundary and some of these reflections eventually reach the listening position or listening area together with the sound energy received directly from the loudspeaker. Whereas some of this reflected acoustic energy contributes in a positive sense to the overall sound perception at the listening position, other reflections have been found to be generally problematic, leading for instance to undesired comb-filter effects that affect the timbre of the sound negatively. It has specifically been found that reflections from the portions of the floor and ceiling between the loudspeaker and the listening position are generally undesirable, and that they should at least be suitably attenuated as compared to the direct sound from the loudspeaker. Also reflections from a wall or other spacially extended obstacle behind the loudspeaker will often lead to the above mentioned undesirable comb-filter effects.
It has furthermore been recognized that there should be a balance between the sound received at the listening position directly from the loudspeaker and the reverberant sound, i.e. the sound caused by reflections. In a typical loudspeaker set-up in a listening room the level of the direct sound and of the reverberant sound are on the same order of magnitude. If the above mentioned undesirable effects of some of the reflections in the room were not taken into consideration, a uniform radiation from the loudspeaker in all directions should thus be aimed at. It is however apparent from the above that a suitable compromise between this omnidirectional radiation of sound energy and attenuation of radiation in some directions must be considered, for instance by tailoring the directivity of the loudspeakerxe2x80x94or of the different loudspeaker units (treble unit, mid-frequency unit, etc.) in loudspeaker systems comprising more than one radiating unit.
Means of tailoring the directivity of loudspeakers are numerous within the art of electroacoustics and have been described regularly at least since the 1930""s. Such means have generally comprised various forms of acrostic lenses or either plane or curved reflector surfaces placed in front of a loudspeaker driver diaphragm. See, for example, U.S. Pat. No. 5,615,176, U.S. Pat. No. 6,068,080 and U.S. Pat. No. 6,435,301, each to the present inventor. See, also, U.S. Pat. No. 4,836,329 to Klayman and UK Patent No. 830,745 to Quennell. Each of these references are incorporated by reference in their entirety herein.
Based on the above background it is an advantage of the present invention to provide a sound reproduction system which does not suffer from the above mentioned drawbacks relating to unwanted reflections and the resulting comb-filter effects but which on the other hand still maintains a broad and uniform directional characteristic throughout the region in the listening room in which listening positions are located.
Specifically the device according to embodiments of the invention should provide attenuation of typical reflections from the floor and ceiling between the device and the listening position and of the reflections from boundaries or obstacles behind the device.
The acoustic requirements of such a device can be broadly reformulated by requiring that the device must minimize the reflected sound from those surfaces (i.e. room boundaries or surfaces of obstacles in the room, the dimensions of which are large enough compared with the wavelength of the radiated sound to cause appreciable reflections) that result in essentially the same interaural difference of the reflected sound from that particular surface and of the sound received directly from the device. Those reflections that fulfill the above requirement are the reflections which are most likely to give rise to the above mentioned undesired comb-filter effects.
The above requirement is illustrated in FIGS. 1a and 1b. Thus FIG. 1a shows a horizontal cross-section through a listening room, a sound source (for instance the device according to the invention) and a listener placed in front of the sound source. The sound received directly from the source at the two ears of the listener is indicated by the arrows D whereas sound reflected from the left wall of the room is indicated by R. The interaural difference (both time and intensity differences as a function of frequency) of the direct sound D is close to zero at all frequencies whereas the interaural difference of the reflected sound R is substantially different from zero. The corresponding interaural time difference will be different from zero at all frequencies whereas the interaural intensity difference will tend to increase with frequency. Reflections of this kind are not attenuated by the device according to the invention as defined by the above requirement.
Referring now to FIG. 1B, there is shown a vertical cross-section through the listening room and the sound source and listener are shown together with the direct sound D. The reflections from the floor, Rf, from the ceiling, Rc, and from the wall behind the sound source, Rb, are also shown. The interaural differences of each of the above three reflections will be approximately equal to the interaural difference of the direct sound, i.e. in this specific case approximately equal to zero.
According to embodiments of the invention, the above requirements based on the interaural differences are fulfilled by providing a sound reproduction device having a substantially uniform directivity in the horizontal plane through the device in front of the device from approximately xe2x88x9290 degrees azimuth angle to +90 degrees azimuth angle, a substantial attenuation of the directivity in the horizontal plane through the device at the back of the device from approximately +90 degrees azimuth through 180 degrees azimuth to approximately xe2x88x9290 degrees azimuth, and a directivity in the vertical plane through the device which exhibits attenuation in those directions of sound radiation which are likely to give rise to said undesired reflections from the floor and the ceiling. Various examples of measurements carried out on a specific embodiment of a reproduction device according to the invention are shown in FIGS. 5A through 5D and in FIGS. 6A through 6D.
According to the invention there is thus provided a sound reproduction device having a directivity which can be tailored according to the above requirements and, if necessary, to further requirements of a specific listening room. Embodiments of the device according to the invention thus include:
one or more generators of sound energy for delivering sound energy to the listening position(s) or a listening area in a room, and
means for directing portions of said sound energy from said one or more generators to said listening position(s)/listening region,
where said means for directing sound energy are adapted for minimizing the reflected sound from each of one or more surfaces that results in essentially the same interaural difference of the reflected sound and of the sound received directly from said means for directing sound energy.
One example of such means for directing sound would be acoustic lenses or reflectors of various kinds and the embodiment of the invention described in the detailed description of the invention is in fact based on a further development of an acoustic reflector disclosed in U.S. Pat. No. 5,615,176 and U.S. Pat. No. 6,068,080. It is however understood, that other kinds of acoustic reflectors or lenses or alternatively arrays of a plurality of sound sources could also be used to carry out the above inventive principle without thereby departing from the invention as defined in the appended claims.
According to embodiments of the invention, a plurality of means for directing sound may be used in a single reproduction device according to the invention. In order to optimize such means according to the specific wavelengths of sound to be handled by that specific means, the overall dimensions thereof, and possibly also other pertinent acoustic parameters such as shape and placement of reflective surfaces, surface structure of various surfaces and placement of acoustic attenuation material etc., it is in principle advantageous to apply more than one of such means and optimize the individual characteristics thereof. This provides for the further possibility if desired to use different directional characteristics of the different means, for instancexe2x80x94in case of acoustic reflectorsxe2x80x94to apply different orientations of these relative to the surroundings. It could also well be beneficial to utilize different kinds of acoustic generators in the different means, for instance according to different requirements relating to the radiated frequency ranges and the radiated acoustic power in each different frequency range.
It is also possible to combine the sound reproduction device according to the invention, which hence fulfills the above requirements relating to interaural differences, with other sound reproduction devices that are not designed to meet these requirements. For instance a combination of the device according to the inventionxe2x80x94mainly intended for reproduction of higher frequencies, for instance above 500 Hzxe2x80x94with an essentially omnidirectional device for low frequency reproduction could in practice be utilized to advantage.